The present invention relates generally to devices used to temporarily grip and hold pipe used in the drilling and completion of wells. More specifically, the present invention relates to an assembly for temporarily gripping and supporting a well pipe in a manner to minimize damage to the pipe caused by the gripping mechanism.
Slip assemblies are customarily employed to temporarily grip and hold pipe as it is being run into or pulled from a well. In a conventional slip assembly, tapered slips, which are carried in a tapered slip bowl, are "set" into gripping engagement with the pipe extending through the center of the bowl by moving the slips into contact with the pipe and then slightly lowering the pipe to allow the slips to support the pipe weight. The surface friction between the slips and the pipe causes the slips to move with the pipe, which pushes the tapered slips axially downwardly into the tapered slip bowl. This relative movement between the tapered slips and the tapered bowl forces the slips radially toward each other to grip the pipe extending through the center of the assembly. As the weight of the string increases, the downward force on the slips increases, which, in turn, acts through the engaged tapered surfaces to increase the radial pipe gripping force exerted by the slips. The slips are released by first lifting the string to relieve the weight on the slips and then retracting the slips out of engagement with the pipe.
The slips are typically equipped with replaceable, steel slip-dies that contact and grip the pipe. Conventional steel dies are typically equipped with radially projecting teeth that are designed to penetrate the outer pipe surface to increase the gripping force of the slips. The usual slip setting procedure can produce die-tooth cuts in the pipe surfaces that decrease the thickness and structural strength of the pipe, provide a corrosion attack point, and otherwise detrimentally affect the pipe.
Efforts at reducing the scarring caused by die teeth include the use of slip dies with very small teeth or specially configured teeth or, in some cases, with no teeth at all. While the prior art designs produce reduced pipe damage, as compared with conventional steel toothed-dies, a primary problem with these designs is that the slips can sometimes fail to grip the pipe securely and thus permit the string to slide through the slip assembly. The problem is most likely to occur as the string weight increases or when the slip teeth become clogged with debris or when the string or slips are contaminated with oil or other slippery substances.
If the pipe string slides through the dies, in many cases, the downward slide is stopped suddenly when a pipe coupling at the end of a pipe joint engages the slip assembly. Such slippage is objectionable in that it allows the string to be mispositioned, and also damages the pipe surface as the pipe slides through the slips. Moreover, if the impact of the coupling striking the slip assembly is strong enough, the coupling may be knocked free of the coupling allowing the string to fall into the well.
One prior art design, described in U.S. Pat. No. 3,579,753, describes a smooth die system that employs a special die carriage design to increase the radial die forces acting on the pipe. The patented system requires a relatively complicated slip carrier design that can be expensive to produce and maintain. No provision is made in the patented system for preventing pipe slippage if the smooth die slips should malfunction.
Other prior art devices for holding pipe without damaging the pipe surface have generally included complex mechanisms that are expensive to build and maintain. These prior art devices also lack an effective backup holding mechanism to prevent pipe movement if the primary holding device fails.